1. Field of the Invention
This invention relates to a control apparatus, a control method, and an engine control unit which control a controlled object with a control algorithm using a controlled object model.
2. Description of the Related Art
Conventionally, as a control apparatus of this kind, the present assignee has already proposed a control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-23922. The control apparatus controls the rotational speed of a driven shaft as a controlled variable by inputting a control input to a clutch mechanism as a controlled object, and includes a controller for calculating the control input. This controller calculates the control input with a target filter-type two-degree-of-freedom sliding mode control algorithm, based on a controlled object model of a discrete-time system, in which the relationship between the control input and the controlled variable is expressed by a recurrence formula. The control input is input to an actuator of the clutch mechanism, and the controlled variable is controlled such that it is caused to converge to its target value.
Further, the controller shown in FIG. 5 of the Japanese Laid-Open Patent Publication (Kokai) No. 2005-23922 includes an identifier that identifies model parameters of the controlled object with an identification algorithm, e.g. based on the sequential least-squares method. When the sequential least-squares method is employed for the identification algorithm for the identifier, the model parameters are identified onboard, and hence even when the controlled object model ceases to match the characteristics of a clutch mechanism as an actual controlled object due to variation between individual products of the clutch mechanism or aging of the same, causing a modeling error, it is possible to control the clutch mechanism while quickly compensating for the modeling error.
In the control apparatus of the Japanese Laid-Open Patent Publication (Kokai) No. 2005-23922 configured as above, since the control input is calculated with a target filter-type two-degree-of-freedom sliding mode control algorithm, it is possible to separately change the rate and behavior of convergence of the controlled variable to the target value for adjustment, which makes it possible to ensure both high-level stability and accuracy of control. Further, when the sequential least-squares method is employed for the identifier, it is possible to control the clutch mechanism while compensating for the modeling error onboard, which makes it possible to further increase the control accuracy.
When the conventional control apparatus described above is applied to a controlled object having characteristics that a controlled variable thereof takes an extremum value (local maximum value or local minimum value) as a control input (hereinafter referred to as “the controlled object having extremal characteristics”) changes, if a target value of the controlled variable is set to a value larger than the local maximum value of the controlled variable or a value smaller than the local minimum value of the same, the controlled variable cannot reach the target value, so that the control input is calculated such that the controlled variable is changed up to the maximum value or the minimum value. As a result, the controlled variable is controlled in a direction largely deviating from the target value. That is, the control system is made unstable, and the accuracy of control is largely degraded.
Further, the identification algorithm, such as one based on the sequential least-squares method used in the above-mentioned identifier, is applicable, insofar as the controlled object model can be expressed by a recurrence formula. However, if the controlled object model cannot be expressed by a recurrence formula, e.g. when the relationship between the control input and the controlled variable in the controlled object is not linear, the identification algorithm is not applicable. In the case of such a controlled object, it is impossible to compensate for a modeling error, which can be undesirably caused due to variation between individual units of the controlled object and aging of the same, and hence the control accuracy is further degraded.
Further, when the sequential least-squares method is employed, if the relationship between the control input and the controlled variable enters a steady state in which there does not occur almost any change, self-exciting conditions cannot be satisfied, which can prevent proper execution of the identification of the model parameters. To avoid this problem, it is sometimes required to add an excitation input to the control input on purpose. In such a case, due to the excitation input as an extra input to the controlled variable, the stability of control can be degraded.